hydroscope — [hī′drə skōp΄] n. [ HYDRO + SCOPE] a device like a periscope, for viewing things at some distance below the surface of water … English World dictionary

Hydroscope — The word hydroscope is used to mean any of several instruments related to water. One kind is an instrument for seeing below the surface of water, such as a long tube fitted with various lenses so arranged that objects lying at the bottom can be… … Wikipedia